1. Technical Field
This invention generally relates to satellite systems, and more specifically to a satellite orbit monitoring system which determines and maintains accurate satellite orbit data using as few as one or two ground stations. The invention also relates to providing a common satellite position and time correction data update signal applicable to all satellites in a constellation.
2. Background Art
A Global Positioning System (GPS) is a space-based radio positioning network designed to provide users who are equipped with a suitable receiver with position, velocity, and time (PVT) information. Developed by the United States Department of Defense (DOD), the space-based portion of GPS comprises a constellation of GPS satellites in non-geosynchronous 12 hour orbits around the Earth.
FIG. 1 shows an exemplary constellation 100 of GPS satellites 101 in orbit around the Earth. The GPS satellites 101 are located in six orbital planes 102 with four of the GPS satellites 101 in each plane, plus a number of “on orbit” spare satellites (not shown) for redundancy. The orbital planes 102 of the GPS satellites 101 conventionally have an inclination of 55 degrees relative to the equator and an altitude of approximately 20,200 km (10,900 miles); completing an orbit in approximately 12 hours. This configuration positions the GPS satellites 101 so that a minimum of five of the GPS satellites 101 are normally observable (above the horizon) by a GPS receiver on Earth at any given time.
A GPS PVT information calculation is based upon a concept referred to as time of arrival (TAO) ranging. The orbiting GPS satellites 101 each broadcast spread spectrum L Band signals such as, for example, L1 at 1575.42 MHz, L2 at 1227.6 MHz and L5 at 1176 MHz, encoded with the satellite ephemeris (satellite positioning data in an Earth-centered-inertial, coordinate system). A GPS receiver receives the signals from a minimum of four satellites and calculates position and time within a margin of error. The GPS receiver may be included within a handheld receiver, a monitoring station, another satellite, or other application where PVT information is useful.
Using conventional satellite positioning systems to generate satellite PVT information for each of the satellites to transmit to GPS receivers, a minimum of four geographically distant monitoring stations are required, and additional geographically distant monitoring stations, i.e. seven or more, are desirable. The four stations are required to measure one each of three position coordinates for each satellite and the time of each satellite. Because four stations are not always available to a satellite, estimations are sometimes made using the assumption that a satellite's orbit is smooth. Nevertheless, the quality and accuracy of the PVT information is related to the number of monitoring stations available. However, because the satellites are spaced from the Earth at approximately 6 times the Earth's radius, the observation geometry between a satellite and four monitoring stations at the same time is not ideal. Each of those monitoring stations monitors its pseudo range from a plurality of satellites with respect to the monitoring station's fixed and known position on Earth, and together they calculate PVT information for the satellites and transmit this information to each of the satellites individually.
With the requirement of conventional positioning systems that at least four ground monitoring stations be operable and accessible at all times to maintain an accurate GPS signal, there is a significant risk that a country's ability to rely upon its GPS system may be compromised if fewer than four monitoring stations are operable at one time. For example, the United States, which relies heavily upon GPS information with its defensive technologies, presently controls seven ground monitoring stations and would need to expend the resources to completely protect at least four of those to be able to continue defending the remainder of the country. If more than three of the seven monitoring stations were destroyed or damaged such that they could not supply the required daily data, the PVT information used by the satellites would degrade and become completely unreliable over time.
Recent technology advances in satellite positioning systems using the GPS PVT information signals have also employed crosslinking technologies, also called inter-satellite communications, which allow the satellites to communicate with each other as well as determine the relative distances between satellites. Examples of crosslinks which determine relative satellite positioning are shown and described in U.S. Pat. No. 5,979,830 to Kellermeier (Nov. 9, 1999) and U.S. Pat. No. 6,219,617 B1 to Dreischer et al. (Apr. 17, 2001), the disclosures of which are hereby incorporated herein by reference. In each of these patents, a ground monitoring station communicates PVT information to a master satellite which is central to a constellation of satellites. The master satellite, using crosslinks, determines the relative positions of each of the satellites under its control, and communicates position correction instructions to each satellite. This configuration of satellites has also been called the “anchor satellite” configuration. The master satellite, whose orbital position has been determined, acts as an anchor to coordinate the relative position corrections among the other satellites in the constellation and checks that the relative satellite positions are corrected to correspond with the appropriate positions of the satellites in the constellation. The anchor satellite configuration, however, still requires established GPS PVT information to be transmitted to the anchor satellite's receiver for the positioning configuration to properly position the satellites. Thus, the anchor satellite must still reference at least four monitoring stations to obtain accurate GPS PVT information.